During drilling operations, a weighted fluid or mud is generally introduced into the borehole through the interior of the drill string and exits the drill string at the bit. This mud serves several purposes. It is used to cool the drill bit, which undergoes ambient and frictional heating during drilling operations. It is also used to flush drill cuttings back to the surface. One of the most important aspects of drilling mud is its use in formation pressure control. The hydrostatic head of the mud exerts pressure against the borehole wall, preventing formation fluids from entering the borehole during drilling operations. This type of drilling condition is generally referred to as overbalanced drilling. The fluid further contains particles that will build up on the borehole wall, forming a mudcake. The mudcake helps provide structural integrity to the borehole. It further reduces the loss of formation fluids into the borehole. However, the particles that make up the mudcake are transported by fluid at a pressure greater than the formation borehole pressure. As a result, filtrates of the weighted fluid create an invaded zone by contaminating or displacing the formation fluids. This invaded zone may adversely affect formation evaluation. Overbalanced drilling represents the most common drilling regime.
In certain types of formations, especially those with low porosity and permeability, overbalanced drilling can adversely affect the producibility of the formation. A different technique known as underbalanced drilling is used in such instances. In underbalanced drilling, the hydrostatic pressure exerted by the drilling fluid is less than the formation pressure. Under the right formation permeability and porosity conditions, formation hydrocarbons enter the wellbore and are transported to the surface with the drilling fluid, where they are separated out as production.
One of the key producibility parameters is permeability, or a measurement of the formation's ability to permit the transmission or passage of fluids. Permeability is currently measured a number of different ways. One method of doing so is to make sidewall corings of the formation and subject these to known laboratory tests to determine permeability. A more common means is through the use of formation test tools. These tools are placed in contact with the fluidic contact with the formation by means of a conduit. A piston, pump of other mechanism is used to reduce the local formation pressure through the conduit and the formation pressure is allowed to build back up, from which permeability can be estimated. However, the formation tests can result take a considerable amount of time. Moreover, the models used to derive formation permeability make certain assumptions that may prove inaccurate. Thus, there exists a need for a method and apparatus capable of making rapid determinations of permeability.
Characterizing the rate of flow and the type of hydrocarbons produced can be accomplished utilizing NMR logging. NMR logging utilizes techniques directed to aligning the magnetic moments hydrogen nuclei, present in both hydrocarbons and water, and modifying the alignment of spins, the nuclei giving off signals that may be detected. NMR tools, such as the NUMAR MRIL®-PRIME manufactured by Halliburton Company and the CMR™ manufactured by Schlumberger Ltd. are capable of providing formation information, including porosity, permeability, and volume fractions of various fluids in the formation and the types of fluids. However, these tools are both wireline tools that require a cessation of drilling operations, removal of the drill string and the subsequent lowering of the tool and commencement of logging operations. Recent advances in tool design have resulted in NMR tools which have been incorporated into drill strings to permit NMR logging while drilling. Structures for these types of NMR LWD tools have been disclosed in U.S. Pat. No. 5,280,243 to Miller and U.S. Pat. No. 5,557,201 to Kleinberg and U.S. Pat. No. 6,531,868 to Prammer. Halliburton, and Schlumberger now offer commercial NMR LWD services with their respective NMR LWD tools. Halliburton's MRIL-WD™ tool and Schlumberger's proVISION tool are directly capable of making hydrogen T1 measurements while drilling to determine total formation, free fluid and bound fluid porosity. When the drill string is stationary, the tool is capable of making T2 measurements to determine total, free-fluid and capillary bound water, clay-bound water porosity, derive permeability and type hydrocarbons in the formation.